Tire Sidewall

ABSTRACT

The invention relates to a tyre sidewall, having a rubber composition based on at least one diene elastomer, a reinforcing filler comprising carbon black and a vulcanization system, characterized in that the carbon black has a CTAB specific surface area greater than or equal to 90 m 2 /g and that the proportion of this carbon black is between 30 and 40 phr.

The present invention relates to a tyre having sidewalls based on arubber composition, and more particularly to a tyre intended to befitted onto vehicles carrying heavy loads and running at a sustainedspeed, such as, for example, lorries, tractor units, trailers or buses.

Certain current “road” tyres are designed to run at high speed for everlonger distances, because of the improvement in road networks and theexpansion of motorway networks throughout the world. However, since fuelsavings and the need to protect the environment have become a priority,it is important for the tyres to have a low energy consumption. One ofthe sources of energy dissipation is the heating of the tyre, especiallydue to the flexural stresses to which a tyre is subjected and moreparticularly to which the sidewalls of the tyre are subjected.

These prolonged static or dynamic stresses of the sidewalls, in thepresence of ozone make weather checking or cracks that are more or lesspronounced appear, the propagation of which under the effect of thepersistence of the stresses may give rise to significant damage of theside wall in question.

In order to overcome the above drawback, while maintaining theproperties of the sidewall in terms of low rolling resistance, good wearresistance of the sidewall with respect to scraping against pavements,good reinforcement, and conduction, the applicant has surprisinglydiscovered that tyre sidewalls based on rubber compositions thatinclude, as a reinforcing filler, “fine” carbon blacks, that is to saycarbon blacks that have a CTAB specific surface area greater than orequal to 90 m²/g, made it possible to overcome the aforementioneddrawback while retaining a good compromise of all of the properties ofthe sidewall.

“Conventional” compositions for tyre sidewalls are especially describedin Table 1 of Patent EP 1 097 966.

As numerous documents illustrate, among which mention may be made ofdocuments EP 1 231 080, EP 1 526 005 and U.S. Pat. No. 7,105,595, it isknown for a person skilled in the art to use more preferentially for themanufacture of sidewalls, rubber compositions based on a blend ofnatural rubber and synthetic rubber, “coarse” carbon blacks thatcorrespond to an ASTM grade 300 to 700, that is to say carbon blackshaving a CTAB specific surface area of less than 90 m²/g.

Contrary to the a priori knowledge of a person skilled in the art whowould seek, in order to lower hysteresis of an elastomer composition fora tyre sidewall, to use coarser reinforcing fillers by reducing thevolume of filler in the formulation, the applicant has discovered that,on the contrary, the use of finer fillers made it possible to lower therolling resistance while maintaining the other performances of thesidewall.

The invention relates to a tyre sidewall, having a rubber compositionbased on at least one diene elastomer, a reinforcing filler comprisingcarbon black and a vulcanization system, characterized in that thecarbon black has a CTAB specific surface area greater than or equal to90 m²/g and that the proportion of this carbon black is between 30 and40 phr, preferably greater than or equal to 33 phr and morepreferentially still greater than or equal to 35 phr.

Advantageously, the rubber composition comprises from 35 to 55% ofnatural rubber or of synthetic polyisoprene, and from 65 to 45% of adiene elastomer chosen from polybutadiene, styrene-butadiene copolymers,isoprene-butadiene copolymers, isoprene-styrene copolymers andisoprene-butadiene-styrene copolymers.

Another subject of the invention is a process for preparing a tyresidewall composition based on at least one diene elastomer, areinforcing filler comprising carbon black and a vulcanization system,characterized in that the carbon black has a CTAB specific surface areagreater than or equal to 90 m²/g and that the proportion of this carbonblack is between 30 and 40 phr, and which comprises the following steps:

-   -   incorporating the carbon black into the diene elastomer by        thermomechanically mixing all the ingredients, one or more        times, until a maximum temperature of between 110° C. and        190° C. is reached;    -   cooling the mixture down to a temperature below 100° C.;    -   then incorporating the vulcanization system; and    -   mixing everything until a maximum temperature below 110° C. is        reached.

The invention also relates to a tyre comprising a sidewall having arubber composition based on at least one diene elastomer, a reinforcingfiller comprising carbon black and a vulcanization system, characterizedin that the carbon black has a CTAB specific surface area greater thanor equal to 90 m²/g and that the proportion of this carbon black isbetween 30 and 40 phr, and in particular a tyre intended to be fittedonto heavy vehicles or passenger vehicles.

I. MEASUREMENTS AND TESTS USED

The rubber compositions are characterized, before and after curing, asindicated below.

I-1 Rolling Resistance

The rolling resistance is measured according to the ISO 9948 standard(for heavy vehicle tyres) and enables the measurement of the energydissipated when the tyre rests against a flywheel driven by a motor andwhich therefore breaks the rotation of this motor.

I-2 Crack Performance

A tyre is made to roll on a flywheel for 50 km at constant pressure andload, in which tyre an initial notch having a length of 20 mm and adepth of 1 mm has been made on the sidewall, which propagates. Theperformance index is the ratio between the length propagated at thesurface of the control tyre over the length propagated at the surface ofthe tyre to be measured. Since the control has at the start an index of100, a value above 100 indicates a better performance in terms of crackresistance.

II—DETAILED DESCRIPTION OF THE INVENTION

In the present description, unless expressly indicated otherwise, allthe percentages (%) shown are % by weight. Moreover, any interval ofvalues denoted by the expression “between a and b” represents the rangeof values extending from greater than a to less than b (i.e., limits aand b excluded), whereas any interval of values denoted by theexpression “from a to b” means the range of values extending from a upto b (i.e., including the strict limits a and b).

II-1. Diene Elastomer

The term “diene” elastomer or rubber should be understood as meaning, ina known manner, an (one or more are understood) elastomer resulting atleast in part (i.e., a homopolymer or a copolymer) from diene monomers(monomers bearing two carbon-carbon double bonds which may or may not beconjugated).

These diene elastomers can be classified into two categories:“essentially unsaturated” or “essentially saturated”. The term“essentially unsaturated” is understood to mean in general a dieneelastomer resulting at least in part from conjugated diene monomershaving a content of units of diene origin (conjugated dienes) which isgreater than 15% (mol %); thus it is that diene elastomers such as butylrubbers or copolymers of dienes and of α-olefins of EPDM type do notcome within the preceding definition and can in particular be describedas “essentially saturated” diene elastomers (low or very low content ofunits of diene origin, always less than 15%). In the category of“essentially unsaturated” diene elastomers, the term “highlyunsaturated” diene elastomer is understood to mean in particular a dieneelastomer having a content of units of diene origin (conjugated dienes)which is greater than 50%.

Given these definitions, the term diene elastomer capable of being usedin the compositions in accordance with the invention is understood moreparticularly to mean:

-   (a)—any homopolymer obtained by polymerization of a conjugated diene    monomer having from 4 to 12 carbon atoms;-   (b)—any copolymer obtained by copolymerization of one or more    conjugated dienes with one another or with one or more vinylaromatic    compounds having from 8 to 20 carbon atoms;-   (c)—a ternary copolymer obtained by copolymerization of ethylene and    of an α-olefin having 3 to 6 carbon atoms with a non-conjugated    diene monomer having from 6 to 12 carbon atoms, such as, for    example, the elastomers obtained from ethylene and propylene with a    non-conjugated diene monomer of the abovementioned type, such as, in    particular, 1,4-hexadiene, ethylidene norbornene or    dicyclopentadiene;-   (d)—a copolymer of isobutene and of isoprene (butyl rubber) and also    the halogenated versions, in particular chlorinated or brominated    versions, of this type of copolymer.

Although it applies to any type of diene elastomer, a person skilled inthe art of tyres will understand that the present invention ispreferably employed with essentially unsaturated diene elastomers, inparticular of the type (a) or (b) above.

The following are suitable in particular as conjugated dienes:1,3-butadiene, 2-methyl-1,3-butadiene, 2,3-di(C₁-C₅alkyl)-1,3-butadienes, such as, for example, 2,3-dimethyl-1,3-butadiene,2,3-diethyl-1,3-butadiene, 2-methyl-3-ethyl-1,3-butadiene or2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene,1,3-pentadiene or 2,4-hexadiene. The following, for example, aresuitable as vinylaromatic compounds: styrene, ortho-, meta- orpara-methylstyrene, the “vinyltoluene” commercial mixture,para-(tert-butyl)styrene, methoxystyrenes, chlorostyrenes,vinylmesitylene, divinylbenzene or vinylnaphthalene.

The copolymers may contain between 99% and 20% by weight of diene unitsand between 1% and 80% by weight of vinylaromatic units. The elastomersmay have any microstructure which depends on the polymerizationconditions used, in particular on the presence or absence of a modifyingand/or randomizing agent and on the amounts of modifying and/orrandomizing agent employed. The elastomers may, for example, be block,random, sequential or microsequential elastomers and may be prepared indispersion or in solution; they may be coupled and/or star-branched orelse functionalized with a coupling and/or star-branching orfunctionalization agent. Mention may be made, for example, for couplingto carbon black, of functional groups comprising a C—Sn bond or aminofunctional groups, such as benzophenone, for example; mention may bemade, for example, for coupling to a reinforcing inorganic filler, suchas silica, of silanol or polysiloxane functional groups having a silanolend (such as described, for example, in FR 2 740 778 or U.S. Pat. No.6,013,718), alkoxysilane groups (such as described, for example, in FR 2765 882 or U.S. Pat. No. 5,977,238), carboxyl groups (such as described,for example, in WO 01/92402 or U.S. Pat. No. 6,815,473, WO 2004/096865or US 2006/0089445) or polyether groups (such as described, for example,in EP 1 127 909 or U.S. Pat. No. 6,503,973). Mention may also be made,as other examples of functionalized elastomers, of elastomers (such asSBR, BR, NR or IR) of the epoxidized type.

The following are suitable: polybutadienes, in particular those having acontent (molar %) of 1,2-units of between 4% and 80% or those having acontent (molar %) of cis-1,4-units of greater than 80%, polyisoprenes,butadiene/styrene copolymers and in particular those having a T_(g)(glass transition temperature, measured according to Standard ASTMD3418) of between 0° C. and −70° C. and more particularly between −10°C. and −60° C., a styrene content of between 5% and 60% by weight andmore particularly between 20% and 50%, a content (molar %) of 1,2-bondsof the butadiene part of between 4% and 75% and a content (molar %) oftrans-1,4-bonds of between 10% and 80%, butadiene/isoprene copolymers,in particular those having an isoprene content of between 5% and 90% byweight and a T_(g) of −40° C. to −80° C., or isoprene/styrenecopolymers, in particular those having a styrene content of between 5%and 50% by weight and a T_(g) of between −25° C. and −50° C. In the caseof butadiene/styrene/isoprene copolymers, those having a styrene contentof between 5% and 50% by weight and more particularly of between 10% and40%, an isoprene content of between 15% and 60% by weight and moreparticularly between 20% and 50%, a butadiene content of between 5% and50% by weight and more particularly of between 20% and 40%, a content(molar %) of 1,2-units of the butadiene part of between 4% and 85%, acontent (molar %) of trans-1,4-units of the butadiene part of between 6%and 80%, a content (molar %) of 1,2- plus 3,4-units of the isoprene partof between 5% and 70% and a content (molar %) of trans-1,4-units of theisoprene part of between 10% and 50%, and more generally anybutadiene/styrene/isoprene copolymer having a T_(g) of between −20° C.and −70° C., are suitable in particular.

To sum up, the diene elastomer of the composition in accordance with theinvention is preferably chosen from the group of the highly unsaturateddiene elastomers consisting of polybutadienes (abbreviated to “BR”),synthetic polyisoprenes (IR), natural rubber (NR), butadiene copolymers,isoprene copolymers and the mixtures of these elastomers. Suchcopolymers are more preferably chosen from the group consisting ofbutadiene/styrene copolymers (SBR), isoprene/butadiene copolymers (BIR),isoprene/styrene copolymers (SIR) and isoprene/butadiene/styrenecopolymers (SBIR).

The term “isoprene elastomer” is understood to mean, in a known manner,an isoprene homopolymer or copolymer, in other words a diene elastomerchosen from the group consisting of natural rubber (NR), syntheticpolyisoprenes (IR), the various copolymers of isoprene and the mixturesof these elastomers. Mention will in particular be made, among isoprenecopolymers, of isobutene/isoprene copolymers (butyl rubber—IIR),isoprene/styrene copolymers (SIR), isoprene/butadiene copolymers (BIR)or isoprene/butadiene/styrene copolymers (SBIR). This isoprene elastomeris preferably natural rubber or a synthetic cis-1,4-polyisoprene; use ispreferably made, among these synthetic polyisoprenes, of thepolyisoprenes having a content (molar %) of cis-1,4-bonds of greaterthan 90%, more preferably still of greater than 98%.

According to one particular embodiment, the composition in accordancewith the invention may contain at least one essentially saturated dieneelastomer, in particular at least one EPDM copolymer or one butyl rubber(optionally chlorinated or brominated), whether these copolymers areused alone or as a blend with highly unsaturated diene elastomers asmentioned above, in particular NR or IR, BR or SBR.

The compositions of the invention may contain a single diene elastomeror a mixture of several diene elastomers, it being possible for thediene elastomer or elastomers to be used in combination with any type ofsynthetic elastomer other than a diene elastomer, indeed even withpolymers other than elastomers, for example thermoplastic polymers.

II-2. Reinforcing Filler

Use is made, as reinforcing filler, of an organic filler constituted bycarbon black. All reinforcing carbon blacks having a CTAB specificsurface area greater than or equal to 90 m²/g are suitable as carbonblacks, which corresponds in particular to carbon blacks of the 100 or200 series (ASTM grades), such as, for example, the N115, N134, N220 orN234 blacks. The carbon blacks might, for example, be alreadyincorporated in the isoprene elastomer in the form of a masterbatch(see, for example, Applications WO 97/36724 or WO 99/16600).

It is specified that the CTAB specific surface area is determinedaccording to the French standard NF T 45-007 of November 1987 (methodB).

One or more other organic fillers may be combined as a blend with thiscarbon black, such as, for example, functionalized polyvinylaromaticorganic fillers as described in Applications WO-A-2006/069792 andWO-A-2006/069793, and/or one or more reinforcing inorganic fillers suchas silica.

The term “reinforcing inorganic filler” should be understood, in thepresent patent application, by definition, as meaning any inorganic ormineral filler (whatever its colour and its origin (natural orsynthetic)), also known as “white filler”, “clear filler” or even“non-black filler”, in contrast to carbon black, capable of reinforcingby itself alone, without means other than an intermediate couplingagent, a rubber composition intended for the manufacture of tyres, inother words capable of replacing, in its reinforcing role, aconventional tyre-grade carbon black; such a filler is generallycharacterized, in a known manner, by the presence of hydroxyl (—OH)groups at its surface.

The physical state under which the reinforcing inorganic filler isprovided is not important, whether it is in the form of a powder, ofmicropearls, of granules, of beads or any other appropriate densifiedform. Of course, the term reinforcing inorganic filler is alsounderstood to mean mixtures of different reinforcing inorganic fillers,in particular of highly dispersible siliceous and/or aluminous fillersas described below.

Mineral fillers of the siliceous type, in particular silica (SiO₂), orof the aluminous type, in particular alumina (Al₂O₃), are suitable inparticular as reinforcing inorganic fillers. The silica used can be anyreinforcing silica known to a person skilled in the art, in particularany precipitated or pyrogenic silica exhibiting a BET surface area and aCTAB specific surface area both of less than 450 m²/g, preferably from30 to 400 m²/g. As highly dispersible precipitated silicas (known asHDSs), mention will be made, for example, of the Ultrasil 7000 andUltrasil 7005 silicas from Degussa, the Zeosil 1165 MP, 1135 MP and 1115MP silicas from Rhodia, the Hi-Sil EZ150G silica from PPG, the Zeopol8715, 8745 and 8755 silicas from Huber or the silicas with a highspecific surface area as described in Application WO 03/16837.

In order to couple the reinforcing inorganic filler to the dieneelastomer, use is made, in a known manner, of an at least bifunctionalcoupling agent (or bonding agent) intended to provide a satisfactoryconnection, of chemical and/or physical nature, between the inorganicfiller (surface of its particles) and the diene elastomer, in particularbifunctional organosilanes or polyorganosiloxanes.

For a tyre sidewall composition in accordance with the invention, use ispreferably made, as reinforcing filler, of carbon black having aspecific surface area greater than or equal to 90 m²/g, in a proportionbetween 30 and 40 phr. The term “between” is understood in the broadsense, that is to say that the proportion of carbon black is greaterthan or equal to 30 phr and less than or equal to 40 phr. Preferably thecontent of carbon black is between 33 phr and 40 phr, and morepreferably between 35 phr and 40 phr.

The carbon black may advantageously constitute the sole reinforcingfiller or the predominant reinforcing filler. Of course, it is possibleto use this sole carbon black or a blend with one or more other carbonblacks having different ASTM grades.

II-3. Various Additives

The rubber compositions in accordance with the invention may alsocomprise all or a portion of the usual additives customarily used inelastomer compositions intended for the manufacture of tyres orsemi-finished products for tyres, such as, for example, otherplasticizing agents (other than the plasticizing system of theinvention), preferably non-aromatic or very slightly aromaticplasticizing agents, for example naphthenic or paraffinic oils, MES orTDAE oils, glycerol esters (in particular trioleates), especiallynatural esters, such as rapeseed or sunflower vegetable oils, pigments,protective agents, such as antiozonants, antioxidants, anti-fatigueagents, crosslinking system based either on sulphur or on sulphur donorsand/or on peroxide and/or on bismaleimides, vulcanization accelerators,vulcanization activators or anti-reversion agents.

These compositions may also contain, in addition to coupling agents,coupling activators, agents for covering the inorganic fillers or moregenerally processing aids capable, in a known manner, by virtue of animprovement in the dispersion of the filler in the rubber matrix and alowering of the viscosity of the compositions, of improving theirability to be processed in the green state, these agents being, forexample, hydrolysable silanes, such as alkylalkoxysilanes, polyols,polyethers, primary, secondary or tertiary amines or hydroxylated orhydrolysable polyorganosiloxanes.

II-4. Manufacture of the Rubber Compositions

The compositions are manufactured in appropriate mixers using twosuccessive preparation phases well known to a person skilled in the art:a first phase of thermomechanical working or kneading at hightemperature, up to a maximum temperature of between 110° C. and 190° C.,preferably between 130° C. and 180° C., followed by a second phase ofmechanical working up to a lower temperature, typically of less than110° C., for example between 40° C. and 100° C., finishing phase duringwhich the crosslinking system is incorporated.

The process in accordance with the invention for preparing a rubbercomposition for a tyre inner liner comprises the following steps:

-   -   incorporating into an elastomer at least one reinforcing filler,        graphite, during a first step, by thermomechanically mixing all        the ingredients, one or more times, until a maximum temperature        of between 110° C. and 190° C. is reached;    -   then incorporating a crosslinking system during a second step;    -   mixing everything until a maximum temperature below 110° C. is        reached.

These two steps may be carried out one after the other using the samemixer or may be separated by a step of cooling down to a temperaturebelow 100° C., the final step then being carried out using a secondmixer.

By way of example, the first phase is carried out in a singlethermomechanical step during which all the necessary base constituents(elastomer, reinforcing filler and coupling agent if necessary andgraphite) are firstly introduced into a suitable mixer, such as astandard internal mixer, and then secondly, for example after one to twominutes of mixing, the other additives, optional covering agents orcomplementary processing aids, with the exception of the crosslinkingsystem, are introduced. After the mixture thus obtained has cooled, thecrosslinking system is then incorporated in an external mixer, such asan open mill, maintained at low temperature (for example between 40° C.and 100° C.). All the ingredients are then mixed for a few minutes, forexample between 2 and 15 minutes.

The crosslinking system is preferably a vulcanization system based onsulphur and on an accelerator. Use may be made of any compound capableof acting as a vulcanization accelerator for elastomers in the presenceof sulphur, in particular those chosen from the group formed by2-mercaptobenzothiazyl disulphide (abbreviated to “MBTS”),N-cyclohexyl-2-benzothiazyl sulphenamide (abbreviated to “CBS”),N,N-dicyclohexyl-2-benzothiazyl sulphenamide (abbreviated to “DCBS”),N-tert-butyl-2-benzothiazyl sulphenamide (abbreviated to “TBBS”),N-tert-butyl-2-benzothiazyl sulphenimide (abbreviated to “TBSI”) andmixtures of these compounds. Preferably, a primary accelerator of thesulphenamide type is used.

Various known secondary accelerators or vulcanization activators, suchas zinc oxide, stearic acid, guanidine derivatives (in particulardiphenylguanidine), etc., are added to this vulcanization system duringthe first phase and/or during the second phase.

The final composition thus obtained is then calendered, for example inthe form of a sheet or a slab, especially for laboratorycharacterization, or else extruded in order to form, for example, arubber profiled element used for producing semi-finished products suchas tyre sidewalls.

The vulcanization (or curing) is carried out, in a known manner,generally at a temperature of between 130° C. and 200° C. for asufficient time, which may vary for example between 5 and 90 minutes,depending in particular on the curing temperature, on the vulcanizationsystem adopted and on the vulcanization kinetics of the composition inquestion.

The following example illustrate the invention without however limitingit.

III. EXAMPLES OF THE IMPLEMENTATION OF THE INVENTION III-1. Preparationof the Rubber Compositions

The tests which follow are carried out in the following way: introducedinto an internal mixer, which is 70% filled and has an initial vesseltemperature of approximately 90° C., are the diene elastomer (SBR and BRblend), the silica topped up with a small amount of carbon black, thecoupling agent, then, after mixing for one to two minutes, the variousother ingredients, with the exception of the vulcanization system.Thermomechanical working (non-productive phase) is then carried out inone step (total mixing time equal to approximately 5 minutes), until amaximum “dropping” temperature of approximately 165° C. is reached. Themixture thus obtained is recovered and cooled and then the coveringagent (when the latter is present) and the vulcanization system (sulphurand sulphenamide accelerator) are incorporated thereinto on an externalmixer (homofinisher) at 70° C., all the ingredients being mixed(productive phase) for approximately 5 to 6 minutes.

The compositions thus obtained are subsequently calendered, either inthe form of slabs (thickness of 2 to 3 mm) or of thin sheets of rubber,for the measurement of their physical or mechanical properties, or inthe form of profiled elements that can be used directly, after cuttingand/or assembling to the desired dimensions, for example assemi-finished products for tyres, in particular as tyre sidewalls.

III-2. Tests

The purpose of this example is to demonstrate the improvement insidewalls of tyres in accordance with the invention compared tosidewalls of a “conventional” control tyre for heavy vehicles.

Tyres of size 315/60R22.5 were produced with sidewalls havingcompositions A, B and C respectively.

The three tyre sidewall compositions A, B and C were manufactured inaccordance with the process explained in detail in the previous section.These compositions, listed in Table 1 below (where the amounts areexpressed in phr or parts by weight per hundred parts of rubber(elastomer)), differ in terms of the nature and amount of theirrespective reinforcing filler, and also in terms of the amount ofplasticizing agent used.

The formulations are given in Table 1 below.

TABLE 1 Composition: A B C NR (1) 50 50 50 BR (2) 50 50 50 Carbon black(3) 55 — — Carbon black (4) — 35 33.5 Silica (5) — — 10 Coupling agent(6) — — 0.5 Plasticizing agent (7) 20 10 10 Wax 1 1 1 Antioxidant (8) 33 3 ZnO 2.5 2.5 2.5 Stearic acid 1.5 1.5 1.5 Sulphur 1.6 1.6 1.6Accelerator (9) 1 1 1 (1) Natural rubber (2) BR with 4.3% of 1,2-; 2.7%of trans-; 93% of cis-1,4-(T_(g) = −106° C.); (3) Carbon black N330having a CTAB specific surface area equal to 82 m²/g; (4) Carbon blackN234 having a CTAB specific surface area equal to 119 m²/g; (5) Silica“Ultrasil VN3” from Degussa; (6) Coupling agent TESPT (“Si69” fromDegussa); (7) MES oil (Flexon 683, Exxon Mobil); (8)N-(1,3-Dimethylbutyl)-N-phenyl-para-phenylenediamine (Santoflex 6-PPDfrom Flexsys); (9) N-cyclohexyl-2-benzothiazyl sulphenamide (SantocureCBS from Flexsys).

Thus, compositions A, B and C are defined as follows:

-   -   control composition A is a “conventional” heavy vehicle tyre        sidewall composition that includes a carbon black of grade 300;    -   composition B, in accordance with the present invention,        comprises a carbon black of grade 200 with a content lower than        that used in the two control compositions;    -   composition C, in accordance with the invention, comprises a        blend of carbon black of grade 200 and silica.

In compositions B and C, the content of plasticizing agent is lower thanthat of the control composition A; indeed, it is clear to a personskilled in the art that in order to obtain a satisfactory stiffness fora composition in which the content of reinforcing filler has beenreduced, it is necessary to also reduce the content of plasticizingagent.

The electrical conductivity results show that the three compositions areconductive (their electrical resistivity measured being less than 10⁶ohm), and the rolling resistance and crack performance tests are givenin Table 2 below.

TABLE 2 Composition: A B C Rolling resistance Control −0.15 kg/t −0.13kg/t Crack performance 100 400 137

It is surprisingly observed that the tyres for which the sidewallscomprise the compositions in accordance with the invention, B and C,comprising carbon black of grade 200, exhibit a substantial improvementin rolling resistance compared to the tyres comprising sidewalls thathave a conventional control composition A.

Furthermore, the improvement in terms of crack performance is also verysubstantial as regards compositions B or C in accordance with theinvention, and more particularly for composition B.

Thus the tyre sidewall compositions comprising carbon blacks having aCTAB specific surface area greater than or equal to 90 m²/g in aproportion between 30 phr and 40 phr therefore make it possible,surprisingly, to improve the rolling resistance and the crackperformance while maintaining the properties of the sidewall, especiallyin terms of conductivity.

1. A tire sidewall, comprising a rubber composition obtained from atleast one diene elastomer, a reinforcing filler comprising carbon blackand a sulphur-based crosslinking system, wherein the carbon black has aCTAB specific surface area greater than or equal to 90 m²/g and whereinthe proportion of this carbon black in said rubber composition isbetween 30 and 40 phr.
 2. A tire sidewall according to claim 1, whereinthe proportion of carbon black is greater than or equal to 33 phr.
 3. Atire sidewall according to either one of claim 1, wherein the proportionof carbon black is greater than or equal to 35 phr.
 4. A tire sidewallaccording to claim 1, wherein the carbon black is blended with one ormore other reinforcing fillers.
 5. A tire sidewall according to claim 4,wherein the carbon black is blended with one or more other organicreinforcing fillers.
 6. A tire sidewall according to claim 4, whereinthe carbon black is blended with one or more inorganic reinforcingfillers.
 7. A tire sidewall according to claim 1, wherein the rubbercomposition is obtained from 35 to 55% of natural rubber or of syntheticpolyisoprene, and from 65 to 45% of a diene elastomer chosen frompolybutadiene, styrene-butadiene copolymers, isoprene-butadienecopolymers, isoprene-styrene copolymers and isoprene-butadiene-styrenecopolymers.
 8. Process for preparing a tire sidewall composition basedon at least one diene elastomer, a reinforcing filler comprising carbonblack and a sulphur-based crosslinking system, wherein in that thecarbon black has a CTAB specific surface area greater than or equal to90 m²/g and wherein the proportion of this carbon black is between 30and 40 phr, and which process comprises: incorporating the carbon blackinto the diene elastomer by thermomechanically mixing all theingredients, one or more times, until a maximum temperature of between110° C. and 190° C. is reached; cooling the mixture down to atemperature below 100° C.; then incorporating the vulcanization system;and mixing everything until a maximum temperature below 110° C. isreached.
 9. A tire comprising a sidewall according to claim
 1. 10. Atire suitable for fitting to passenger vehicles that comprises asidewall according to claim
 1. 11. A tire suitable for fitting to heavyroad vehicles that comprises a sidewall according to claim 1.